There are innumerable situations in which a gas requires to be stored for subsequent release under substantially controlled conditions for practical use to be made of the physical and/or chemical properties of the gas. By way of example, stored and released gas may be employed for pressurised dispensing of a substance from a container using the gas as a propellent.
A number of practical considerations limit the substances which can be used as propellent gases and/or the circumstances in which a given substance can be used as a propellent gas. By way of non-limiting examples, such considerations include the ability to sustain pressure within acceptable limits during use, safety factors which include flammability and toxicity of the propellent, and chemical reactivity of the propellent with the container and, mainly in the case of non-barrier dispensers, reactivity of the propellent with the product to be dispensed. By way of a non-limiting example of the circumstances affecting use of a substance as a propellent gas in a non-barrier dispenser, the substance may be substantially inert with respect to one product but react unfavourably with another product (unless isolated by a barrier).
For many years the substances collectively known as CFC's (chlorofluorocarbons) were popular for use as propellents in pressure pack dispensers owing to favourable pressure characteristics combined with non-flammability and apparent non-toxicity, but CFC's are now perceived as extreme environmental hazards and are the subject of international sanctions; CFC's are no longer acceptable as propellent substances in pressure pack dispensers. Although some readily available gases are free of hazards and are substantially unreactive (for example, nitrogen), gases per se are generally unsuitable for use as propellents in pressure pack dispensers because of unacceptably rapid fall-off of propellent pressure during use of the pressure pack dispenser. Elaborations of construction and use may reduce the unwanted effects of these adverse pressure characteristics, but at the expense of increased complexity and cost, and possibly an increased hazard arising from increased initial internal pressure in the pressure pack dispenser.
Two-phase gas/liquid pressure pack propellent systems may give more acceptable pressure characteristics in terms of an acceptably low fall-off of propellent pressure during use of the pressure pack dispenser, in comparison to a single-phase gas-only system, where the liquid in a two-phase gas/liquid pressure pack propellent system is a pressure-liquefied form of the propellent gas. However the requisite pressure at ambient temperature may be unacceptably high in the context of conventional pressure pack dispensers; additional or alternative disadvantages of two-phase gas/liquefied-gas propellent systems are that they tend to employ gases which are flammable and potential substances of abuse, such as propane, butane and propane/butane mixtures. (It should be noted that such two-phase gas/liquefied gas propellent systems are essentially single-material propellent systems, where the single propellent material is present in both gas and liquid phases; this single material nature is not altered by the propellent being a mixture such as butane and propane, since the components of such mixtures change phase together, and a chemically distinct liquid is not present in such systems.)
To summarise the main considerations for the adoption of a given propellent system in a pressure pack dispenser, the propellent system should be:
(a) free of toxicity over any length of time and in any feasible concentration;
(b) free of environmental hazard over any length of time;
(c) free of other hazards, including but not restricted to hazards of fire and explosion;
(d) maintain adequate dispensing pressure on the product throughout use of the pressure pack dispenser, without excessive pressure at any time;
(e) at least in non-barrier dispensers, be compatible, and preferably non-reactive, with the product to be dispensed; and
(f) be reasonably economic.
The above list of desiderata for a propellent system is only a general indication and is in no way definitive to the exclusion of any other factors; further, the desiderata are not mutually exclusive in the sense that a characteristic of a selected propellent may satisfy two or more desiderata simultaneously (for example, a hypothetical inert substance may be both non-toxic and non-flammable, as in the case of nitrogen).